The invention relates to a plain bearing for a part which is guided longitudinally on a prismatic guide rail. And copending application Ser. No. 445,536, filed Nov. 30, 1982, is a related case to which reference is made for background purposes.
Primarily, plain bearings are used for longitudinal guides since they have the advantage, over anti-friction bearings or air bearings, of being of simple construction. Also, in a few cases, a plain bearing may be specifically selected to achieve a feature which is generally undesired. Thus, a self-locking plain bearing is well suited for guiding the displaceable part of chucking devices, for example, the arm of a center cradle, since in such case there is no need for separate clamping screws to secure the part after clamping the workpiece.
However, plain bearings have a disadvantage which limits their use in guides which must serve with great precision, in that, once the bearing has been clamped by constraining forces which act (tangentially and perpendicularly to the direction of guidance) on the bearing, the bearing no longer returns precisely to its angular-zero position, due to relatively great sliding friction between bearing and guide. If such a bearing is used to guide the arm of a center cradle, in the circumstance in which the offset end of the arm mounts a measurement-reference point to be reproduced with high precision in its position in space, then the length of the arm magnifies the degrading effect of such friction on the accuracy of positioning the reference point. And one cannot always remedy the situation by increasing the longitudinal base, i.e., in the guidance direction.
From West German published patent application No. OS 1,497,258, a carriage guide is known in which the guided part relies on engagement with two cylindrical slide members on the guide rail. The slide members engage in V-grooves in the guided part and have their longitudinal axes aligned parallel to the direction of guidance. But this bearing also, due to its angular position, cannot satisfy demands for increased precision, since the slide members always engage the two walls in their respective grooves and thus cannot be turned without sliding friction.